The offered manufacturer number is an alternative (substitute) to the spare part number you are looking for.
Note: This is a used item! It may show signs of use in the sense of signs of wear, scratches or other signs. Complaints and returns for any of these reasons are therefore excluded.
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Fujitsu S26361-F3697-L615 Memory with 8GB DDR3-1600
Compatible with: Primergy TX150 S8 and similar
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You can install RAM from a different manufacturer, as long as the specifications are the same.
For example, when your computer currently has a DDR3 SO-DIMM module with a speed of 1600MHz installed, you should only use DDR3 SO-DIMMs with a 1600 MHz speed to upgrade/replace your RAM. It is possible to use different frequencies but the system will then use the speed of the slowest RAM module.

From DDR4 onwards, the MHz no longer play a role in the compatibility.
In Windows 10 open the Task-Manager (Control+Shift+Escape), then click on more details in the bottom left corner. Select the tab performance and click on RAM Now you will be able to see many values about your RAM including its speed.

To see how much memory you have installed in your PC, you first need to open the Task-Manager. The you click on "more details" in the bottom left corner. Now choose Performance in the menu on the top, afterwards select Memory from the menu on the left. The you will not only be shown the amount of Memory in your System, but also further details, like how many slots are occupied. If there still is a free slot, you can easily double your memory by installing another one of the same modules already installed.

It could because you installed a 32-Bit operating system, since they can only address up to 4 GB of RAM. Additionally, other system components like your graphics card also need RAM. That is why if you have 4GB installed you won’t be able to utilize every last MB of it.
A 64-bit operating system can address more than 4GB of RAM. So, you can install more than 4GB even in an older Notebook, provided the rest of the hardware supports it too.
There can be many reasons for your operating system crashing but bluescreens are often a clear sign for defective RAM. Error messages like "pci.sys could not be found" also hint at faulty RAM.
You can use a memory test to check your RAM for faults. These can be found on every Ubuntu installation CD under "Memtest".
On www.ubuntu.com you can download an image of this CD, burn it o a CD and boot your PC from it to execute the Memtest on it.
Or you could bring your laptop to a technician, who can fix the fault for you.